Sorghum (Sorghum spp.) is a plant genus comprising approximately 20 species of grasses. It is native to tropical and subtropical regions of East Africa and it is grown in every continent for grain for human and animal consumption, being also used as fodder, and for manufacturing alcoholic beverages. As it is gluten-free, sorghum is suitable for individuals suffering from celiac disease.
Sorghum is more tolerant to drought and excess soil moisture content than most cereals. It is capable of growing properly under varied soil and weather conditions. Likewise, it responds favorably to irrigation, requiring a minimum of 250 mm during its life cycle, with an optimum irrigation ranging from 400-550 mm.
The soil must have an appropriate moisture content at the time of planting in order to achieve a rapid and homogeneous emergence and thereby a good crop implantation. The greatest demand of water starts 30 days after emergence and continues until grains are filled, the most critical stages being those of panicle formation and flowering, as water deficiency at this time will result in decreasing yields.
Furthermore, sorghum has the ability of remaining dormant during periods of drought and resumes growth under favorable periods, although these stress situations may affect performance.
Sorghum requires high temperatures for normal development, and consequently it is more sensitive to low temperatures than other crops. A soil temperature of not less than 18° C. is required for germination; and actual active growth of the plant is not achieved until a temperature of 15° C. is reached, the optimum temperature being of about 32° C.
Acetohydroxyacid synthase (AHAS) is the first enzyme which catalyzes the synthesis of branched amino acids valine, leucine and isoleucine. AHAS is the target of some herbicides such as sulfonylureas, imidazolinones, triazolopyrimidines and pyrimidyloxybenzoates. The two former herbicide families are widely used in modern agriculture due to their low toxicity and high efficacy against weeds.
The imidazolinone family of herbicides comprises imazethapyr, imazaquin, and imazapyr.
Some sulfonylureas present in the market are: metsulfuron-methyl, chlorosulfuron, nicosulfuron, cinosulfuron, imidasulfuron, halosulfuron, rimsulfuron, trisulfuron-methyl, and tribenuron-methyl.
However, there are plants which are resistant to herbicides of the imidazolinone and/or sulfonylurea families; for example, species such as Zea mays, Arabidopsis thaliana, Brassica napus, Glycine max, Nicotiana tabacum, and Oryza sativa (Sebastian et al., (1989) Crop Sci., 29: 1403-1408; Swanson et al., 1989 Theor. Appl. Genet. 78: 525-530; Newhouse et al., (1991) Theor. Appl. Genet., 83: 65-70; Sathasivan et al., (1991) Plant Physiol., 97: 1044-1050; Mourand et al., (1993) J. Heredity 84: 91-96; U.S. Pat. No. 5,545,822). A point mutation has been described in sunflower, in the large subunit of AHAS, which confers resistance to herbicides of the type of imidazolinones (WO 2007/0118920).
Plants resistant to herbicides of the imidazolinone or sulfonylurea type have also been detected. These plants have acquired resistance naturally and have been used for cross-breeding, resulting in herbicide-resistant varieties. From the analysis of resistant plants, a point mutation was determined in the AHAS protein of sunflower resulting in the substitution of the amino acid Ala by Val (White et al., (2003) Weed Sci., 51: 845-853).
In addition, U.S. Pat. Nos. 4,761,373; 5,331,107; 5,304,732; 6,211,438; 6,211,439; and 6,222,100 disclose plants resistant to imidazolinone herbicides. All these patents generally describe the use of an altered AHAS gene to elicit herbicide resistance in plants, and specifically disclose certain imidazolinone resistant corn lines. U.S. Pat. Nos. 5,731,180 and 5,767,361 discuss an isolated gene having a single amino acid substitution in a wild-type monocot AHAS amino acid sequence that results in imidazolinone-specific resistance.
Patent documents WO 2006/007373 and WO 2006/060634 disclose mutations conferring resistance to herbicides of the imidazolinone type in wheat plants.
The publication “Amino acids conferring herbicide resistance in tobacco acetohydroxyacid synthase” describes different point mutations in AHAS that confer herbicide resistance (GM Crops 1: 2, 62-67; Feb. 16, 2010).
Patent document U.S. 20100115663 refers to herbicide-resistant sorghum plants achieved by altering acetyl CoA carboxylase genes. Also disclosed are sorghum plants resistant to the herbicide dinitroaniline (U.S. 20100205686) and sorghum plants resistant to the herbicide acetolactate synthase (WO 2008/073800).